The present invention relates to an actuator for obtaining quantitative motion by utilizing a shape memory effect of a shape memory element and, more particularly, to an actuator wherein the shape memory element is made of a shape memory alloy, and the shape recovery control of the shape memory element is performed by Joule heat generated by a current supplied thereto.
It is known that Ni-Ti and Cu alloys each having a predetermined composition ratio have a shape memory effect. Such a shape memory alloy recovers the memorized shape when it is heated to a predetermined temperature. This property can be effectively used for an element (e.g., various actuators and switches) requiring a mechanical stroke or displacement.
In order to recover the shape of the shape memory element made of a shape memory alloy, heat is applied by any heating means to heat the element to a temperature higher than a reverse critical temperature (i.e., a temperature at which the martensite phase disappears). Various types of heating means can be proposed. In an actuator incorporating a shape memory element, a system is adapted such that a current flows through the shape memory element to recover the shape by the Joule heat. In this current heating system, shape recovery control can be achieved by a pulse current. This system provides a simpler control system than any other system.
In a conventional actuator incorporating a shape memory element made of a shape memory alloy, electrodes are formed at two ends of the shape member element. When the shape is recovered, a current flows to the entire shape memory element through these electrodes, thereby converting the current to Joule heat. The current flowing through the shape memory element can flow substantially uniform throughout the shape memory element. Therefore, heat is also uniformly generated throughout the shape memory element, and the shape recovery caused by the uniform heat can be performed uniformly throughout the shape memory element. In this conventional actuator, the shape memory element as a whole is heated to a temperature exceeding the reverse critical temperature and instantaneously recovers the shape in a single step.
However, in the conventional actuator utilizing the single step of shape recovery, only a single operation is performed, thereby limiting the range of operation and resulting in inconvenience. For example, neither griping, similar to that by an operator's fingers, nor a plurality of complicated operations can be performed by a single actuator. Therefore, the conventional actuator has a limited range of applications.